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Neutrino Physics

Although neutrinos are probably the second most abundant particle in the Universe, their experimental detection happened only in the 50s, tanks to a pioneering experiment by Reines and Cowan, who were subsequently awarded the Nobel Prize. Now, more than fifty years later, we are still investigating some fundamental aspects of these fascinating particles.

Neutrino studies are difficult because these particles have no electrical charge, and interact with other particles only via the 'weak' interaction, one of the four fundamental forces in Nature. Their interaction cross section is extremely small, and makes them able to cross enormous thicknesses of matter. To give an idea of this remarkable property, it suffices to think that the neutrinos generated in the Sun's core travel undisturbed for more than one million kilometers through the solar matter, and create at Earth a flux of about sixty billions of particles per square centimeter per second, an immense flux of which we are totally unaware, unless we build complex and big underground experiments to measure it.

This example illustrates well the twofold nature of neutrino studies. On one hand, natural sources of neutrinos, like the Sun, or the cosmic rays, allow physicists to investigate the nature of these particles. On the other hand, they can be 'messengers' of astrophysical phaenomena, allowing physicists to study the sources emitting them: it is a new and special kind of astronomy. This is why Neutrino Physics is considered part of the research in Astroparticle physics.

The institutional partners of CFA are particularly active in the field of neutrino physics, especially through the experiments carried out in the Gran Sasso National Laboratory (LNGS). Starting from past experiments, MACRO was a huge detector occupying most of Hall B of LNGS. MACRO has been among the first experiments (with Super-Kamiokande) to prove the existence of the phenonemon known as 'neutrino oscillatons', meaning the transformation of one type of neutrino ('flavor') into another type. Neutrinos actually exist in three 'flavors' called electron neutrino, muon neutrino and tau neutrino. In MACRO all the institutional components of CFA participated. MACRO has been decommissioned in 2000, shortly before the end of another historical and very important experiment, Gallex/GNO. This one was initially lead by the Max Planck Institute, while also Tor Vergata, LNGS and L'Aquila University contributed to it. It was a complex radio-chemical experiment, using a huge mass of Gallium as a target for solar neutrinos. The solar neutrino flux was inferred by the number of Gallium atoms transforming into Germanium atoms via neutrino absorption. Gallex/GNO made the first measurement ever of the lowest energy component of the solar neutrino spectrum. This measurement confirmed a significant deficit in the measured flux, subsequently explained, again, by the oscillation mechanism. Another important experiment carried out at LNGS is the Large Volume Detector (LVD), a first class observatory for supernova neutrinos. LVD has about one kton of active scintillator mass, is in operation since 1992 and it is part of the SNEWS international network for early detection of neutrino bursts candidates.

Further developments in the study of the oscillation mechanisms led to a new genereation of experiments. Concerning solar neutrinos, the experiment Borexino featuring a participation of LNGS researchers, is presently taking real time data on low energy solar neutrinos with 300 tons of ultra pure scintillation. On a different neutrino energy scale, the project CNGS (Cern Neutrinos to Gran Sasso) has been active for three years, sending from CERN an artificial beam of muon neutrinos in which the Opera detector is seeking the appearance of tau neutrinos, with the aim to confirm the nechanism of oscillations. Researchers and professors of LNGS and L'Aquila participate in the Opera collaboration. They also participate in the ICARUS experiment, an ambitious program for the detection of neutrinos of the CNGS beam with a novel technique based on liquid Argon.

A different kind of research is represented by the search for the 'neutrinoless double beta decay' (NDBD). This is a very rare (if existent at all) nuclear process that is possible only if the neutrinos have a mass and if they are 'Majorana' particles, that is they coincide with their antiparticles. We know that first condition (mass) is verified, since the oscillation mechanism is only possible if neutrinos have a mass. Therefore, the discovery of neutrinoless double beta decay would verify the Majorana condition for neutrinos and would give an indication about the value of the neutrino mass.

At the Gran Sasso National Laboratory there are two big programs for the search of NDBD, one is CUORE, in which there is a participation of LNGS researchers. CUORE makes use of Telluriun bi-oxyde crystals, cooled down near absolute zero. At this temperature the crystals are very sensitive partlcle detectors, because any particle interaction ocurring inside them translates into a tiny, but measurable, increase in temperature. The other experiment is GERDA, which instead makes use of Germanium crystals immersed in a bath of liquid Argon.